Industrial process control systems are used to monitor and control industrial processes used to produce or transfer fluids or the like. In such systems, it is typically important to measure “process variables” such as temperatures, pressures, flow rates, and others. Process control transmitters measure such process variables and transmit information related to the measured process variable back to a central location such as a central control room.
One type of process variable transmitter is a pressure transmitter which measures process fluid pressure and provides an output related to the measured pressure. This output may be a pressure, a flow rate, a level of a process fluid, or other process variable that can be derived from the measured pressure. The pressure transmitter is configured to transmit information related to the measured pressure back to the central control room. Transmission is typically provided over a two-wire process control loop, however, other communication techniques are sometimes used.
Generally, the pressure is coupled to the process variable transmitter through some type of process coupling. In many instances, a pressure sensor of the transmitter is fluidically coupled to the process fluid either through an isolation fluid or by direct contact with the process fluid. The pressure of the process fluid causes a physical deformation to the pressure sensor which generates an associated electrical change in the pressure sensor such as capacitance or resistance.
A pressure barrier is a mechanical structure that contains process fluid pressure. As such, pressure barriers are key requirements for process fluid pressure measurement system. In order to provide a safe and robust system, some manufacturers provide redundant pressure barriers. Thus, if a primary barrier fails, the process fluid is still contained by the secondary barrier.
One particularly challenging environment for pressure measurement is applications which have very high working pressure. One such application is the subsea environment. In such applications, the static pressure to which the process equipment is exposed can be quite high. Moreover, the process fluid can corrode many known metals. For example, some subsea applications are now being considered that require a 20,000 psi maximum working pressure (MWP). By requiring a 20,000 psi MWP, manufacturing approval standards typically require the pressure barriers of pressure sensors in such environments to withstand 2.5 times the maximum working pressure. Thus, a pressure barrier in such an application would need to be able to withstand 50,000 psi. The design criteria for pressure barriers are important in that they ensure the integrity of the process. Specifically, if the pressure barrier or barriers fail, it is possible for the process fluid to enter the environment. This is highly undesirable because the process fluid may be flammable or even explosive, or may generally cause environmental contamination. Thus, for subsea applications, it is desirable to provide two pressure barriers between the process fluid and the seawater, or the process fluid and the electronic compartment of the process fluid pressure transmitter.